The particle settling velocity is the argument of separation in such processes of mineral processing as classification or enrichment in jigs. It belongs to the so-called complex arguments because it is the function of particle density and size, i.e. the function of two simple arguments. The affiliation to a given subset is determined by the values of two magnitudes and the distribution of such an argument in a sample is the function of distribution of simple arguments. The paper presents distribution of density and sizes of particles which are applied most often in mineral processing. The largest amount of attention was paid to the distribution of particle settling velocity. The fundamental formulas applied for calculation of settling velocity of particle were presented. Both particle density and size are random variables of fixed distributions. Consequently, the particle velocity as a function of simple arguments, i.e. particle density and size, will be also the random variable of a distribution which is the function of distributions of simple arguments. Applying the theorems of probability, concerning distributions of function of random variables, the authors presented general formulas of probability density function of settling velocity for three conditions of particle motion: laminar, turbulent and intermediate, respectively to the formulas of settling velocity, applied in these conditions. As an example, the authors present a detailed calculated distribution of settling velocity for linear forms of frequency functions of particle size and density. The shape of the distribution function of settling velocity depends on the conditions of particle motion.